1. Technical Field
Embodiments of the present disclosure generally relate to a self-refresh control device, and more particularly, to a technology for reducing a self-refresh current.
2. Related Art
It may be desirable to use high-capacity dynamic random access memories (DRAMs) in mobile electronic appliances. The mobile electronic appliances may include smartphones or the like. Generally, data stored in memory cells of a semiconductor memory device such as DRAM may be changed by a leakage current. Therefore, a refresh operation for periodically recharging data stored in the memory cells is needed.
A memory cell of a dynamic semiconductor memory, such as a DRAM, stores data in a capacitive element. Due to leakage of charges from the capacitive element, the memory cell must be periodically refreshed. The refresh process performs the read operation for restoring a level of charges stored in the memory cell to an original state.
Specifically, a semiconductor memory device, such as Double Data Rate Synchronous DRAM (DDR SDRAM), includes a plurality of memory banks for storing data therein. Each memory bank may include tens of millions of memory cells therein. Each memory cell includes a cell capacitor and a cell transistor. The semiconductor memory device may charge or discharge the cell capacitor to store data therein.
The amount of charges stored in the cell capacitor must be, ideally, constant in so far as an additional control signal is not used. However, the amount of charges stored in the cell capacitor is unavoidably changed due to a difference in voltage between the cell capacitor and the peripheral circuit.
In other words, charges may be leaked outside under the condition that the cell capacitor is charged with electricity, or charges may be received under the condition that the cell capacitor is discharged. Changing the amount of charges stored in the cell capacitor may indicate that data stored in the cell capacitor is changed, resulting in loss of the stored data. The semiconductor memory device may perform the refresh operation to prevent the stored data from being lost.
Different types of refresh methods have been developed. Generally, the auto refresh method uses a refresh timer located outside of a memory chip, such that the memory chip can perform the refresh operation in response to a periodic refresh command from a controller.
The self refresh method uses a refresh timer located inside the memory chip, such that all the memory chips are configured to request a refresh start command from the controller.